In some integrated circuits, for example pipeline ADCs (analog-to-digital converters), operational amplifiers (opamps) are used and shared by a plurality of the device units operating in time delayed phases. Opamp sharing saves opamp power and die area, since a shared opamp is used by several device units.
Opamp sharing may introduce additional parasitic capacitances between the device units associated to the shared opamp owing to enhanced routing. Further, the technique of opamp sharing may cause intersymbol interference. This unwanted effect is caused by input and parasitic capacitances of the opamp resulting in that residual charge from a previous sampling phase may still be present in the actual sampling phase in which the opamp is used by another device unit.
As the main contribution to such residual charge comes from the input capacitance of the opamp, intersymbol interference can be minimized by using an opamp with a small input capacitance. However, for some applications, for instance analog-to-digital conversion in a pipeline ADC, high input capacitances occur when high sampling rates are aspired. Therefore, opamp sharing techniques are typically applied only to moderate sampling rates.